The present invention relates generally to an ink jet recording system. More specifically, the invention relates to an ink-jet recording system for recording characters and pictures by causing the flight of at least a coloring material of a liquid ink, which contains the coloring material dispersed in a solvent, as an ink droplet onto a recording medium by electrostatic force.
A recording device for recording a picture by forming recording dots on a recording medium by spraying the recording medium with a liquid ink as micro-droplets called ink droplets has been put to practical use as an ink jet printer. This type of ink jet printer has the advantage of making fewer noises than a mechanical recording device, such as a wire dot printer, or the like. In comparison with an electrophotographic recording device such as Carlson process, there is also an advantage in that it is not required to carry out treatments such as development and fixing. For that reason, the ink jet printer has been noticed as plain paper recording technique.
To date, various ink jet printers have been proposed. Typical systems of ink jet printers include: (a) an electric thermal conversion system for causing the flight of an ink droplet by steam pressure produced by the heat of a heating element (see, e.g., Japanese Patent Publication Nos. 56-9429 and 61-59911), and (b) a piezoelectric system for causing the flight of an ink droplet by mechanical pressure pulses produced by a piezoelectric element (see Japanese Patent Publication No. 53-12138).
As a recording head used for an ink jet printer (which will be hereinafter referred to as an "ink jet head"), a serial scanning head loaded on a carriage for carrying out the recording while moving in a direction (which will be hereinafter referred to as a "main scanning direction") perpendicular to a recording-paper carrying direction (which will be hereinafter referred to as a "feed direction") has been put to practical use. Since this serial scanning head can not carry out the recording unless it is moved by a predetermined distance in the main scanning direction, it is difficult to increase the recording speed. Therefore, a line scanning printer, wherein the recording speed is increased by means of a long recording head having a length which is set to be substantially the same as the width of a recording paper, has been proposed. However, it is not easy to put such a line scanning printer to practical use for the following reasons.
In the ink jet recording system, a large number of separate small nozzles corresponding to resolution are provided, and ink is essentially concentrated easily due to evaporation and volatilization of the solvent, so that the nozzles clog easily. In addition, in the system for using the steam pressure to form an ink jet, an insoluble matter formed by thermally or chemically reacting with the ink is adhered to the nozzle, so that the nozzles clog easily. Moreover, in the system for using the pressure caused by a piezoelectric element, the nozzles clog more easily due to the complicated structure, such as ink passages. In the line scanning head which uses a larger number of nozzles than serial scanning head using tens through one hundred and tens of nozzles, i.e., which uses thousands of nozzles, the nozzles clog with higher probability, so that there is no reliability from a practical standpoint.
Moreover, in the system for using the steam pressure, it is difficult to produce particles, each of which has a particle diameter of not greater than 20 .mu.m corresponding to each of recording dots having a diameter of about 50 .mu.m, so that it is difficult to manufacture a head having a high resolution. In addition, in the case of the system for using the pressure caused by a piezoelectric element, the structure of the recording head is complicated, so that it is difficult to manufacture a head having a high resolution for an issue of working technique. For these reasons, in conventional ink jet devices of any systems, there is a problem in that it is difficult to improve the resolution.
In order to eliminate the problems described above, there has been provided an ink jet recording device, wherein voltage is applied to an array of electrodes, which is formed by arranging a plurality of separate electrodes formed on a substrate using a thin film, to cause the flight of an ink or a coloring material component thereof as an ink droplet from an ink liquid surface using electrostatic force.
Specifically, there have been proposed a system for causing the flight of an ink droplet using electrostatic force (see Japanese Patent Laid-Open Nos. 49-62024 and 56-4467), a system for causing the flight of an ink droplet by increasing the concentration of a coloring material using an ink containing a charged coloring component (see Japanese Patent Laid-Open No. 502218) and so forth. In these systems, a recording head has a slit-like nozzle structure which is not required to provide nozzles for each of separate dots, or a nozzleless structure which is not required to provide partitions of ink passages for each of separate dots. Therefore, these systems are effective in the prevention and restoration of clogging which is an obstacle to the realization of a line scanning recording head. In addition, the latter system for increasing the concentration of a coloring material is suitable for the increase of resolution since it is possible to stably produce an ink droplet of a very small particle to cause the flight thereof.
On the other hand, in the electrostatic attraction ink jet system, it is possible to cause the flight of very fine particles of ink from a common slit-like nozzle without the need of separate nozzles corresponding to respective picture dots, and it is free from the clogging of the nozzle. Utilizing this advantage, there has been provided a multi-ink jet head of an electrostatic attraction system, wherein a large number of electrodes are provided in a slit-like nozzle to form a multihead. Referring to the schematic view of FIG. 1, the electrostatic attraction ink jet system will be described below.
As shown in FIG. 1, a slit 2 is formed in two insulating layers 1, and an oil based conductive ink is carried from an outside ink supply tank 4 to the tip portion 3 of the slit 2 by capillary phenomenon. In the slit 2, a plurality of electrodes 5 are provided. When a bias voltage 6 and a signal voltage 7 corresponding to a picture signal are applied to the respective electrodes 5, ink particles 8 fly from the respective electrodes 5 in response to the picture signal, so that a picture is formed on a recording paper.
In this electrostatic attraction ink jet system, fine ink particles being smaller than the slit width can fly, so that it is possible to prevent the tip of the slit from clogging, and it is also possible to form ink particles of a high resolution.
However, since the ink can freely move on the substrate of the recording head in a main scanning direction, there is a problem in that the ink-droplet emitting position is unstable.
In addition, since the flight of an ink droplet onto a recording paper are caused by a voltage having the same polarity as the charged polarity of a coloring material, the coloring material component is repulsed to escape from the position of the electrode on the recording head, so that there is a problem in that the coloring material component can not be stably supplied to the ink-droplet emitting position. Therefore, it is difficult to stably cause the flight of a sufficient amount of ink droplets from a predetermined emitting point, so that there is a problem in that it is not possible to suitably record characters and pictures on a recording material.